Muted automatic volume control circuits



Dec. l0, 1935. J, 5 STARRETT 2,@239

' MUTED AUTOMATIC VOLUME CONTROL CIRCUITS Original' Filed Nov. 25, 1932 2 Sheets-Sheet 1 www my m35 J. s. sTARRET-n' MUTED AUTOMATIC VOLUME CONTROL CIRCUITS Original lFiled Nov. 25, 193.2 Y 2 Sheets-Sheet 2 LEE v AAAAAA m R om m mr m WMO NJ. n Il v. A

Patented Dec. 10, 1935 UNITED STATES PATENT OFFICE MUTED AUTOMATIC VOLUME CONTROL CIRCUITS John S. Starrett, Wilmette, Ill., assignor to Radio Corporation of America, a corporation of Dela- Ware 11 claims. (c1. 25o- 20) My present invention is a division of application serial No. 644,160, filed November 25, 1932, and relates to automatic volume control circuits for radio receivers, and more particularly to improved circuits for the accomplishment of detection, automatic volume control and automatic suppression, or muting of, interchannel 110158.

There has been disclosed by D. G. Burnside in application serial No. 644,149, filed November 25, 1932, various circuits for securing detection and delayed automatic volume control functions from a single stage. It has also been shown by D. G. Burnside that receiver noises present when no carrier is tuned in could be prevented from appearing in the reproducer by a system of so-called delayed detection, in Whichsystem a detector diode anode is biased negatively by an amount just greater than the noise potential. It Was pointed out that this biasing prevented the diode anode from becoming positive, and therefore, prevented detection; consequently, the noise voltages are not able to pass the detector.

Furthermore, it has also been shown in the aforementioned Burnside docket that the three functions of detection, delayed automatic volume control and inter-channel noise suppression could be accomplished with a single tube of the duplex diode-triode type, a tube of the latter type having been disclosed by T. M. Shrader in application serial No. 622,140, filed July 12, 1932. While the inter-channel noise suppressor arrangement for an automatic volume control circuit shown by Burnside is entirely satisfactory for many purposes, at times, due to the different delay voltages which may be used for biasing the detector and volume control anodes, interaction of an undesirable kind between the two circuits may be experienced. If the aforedescribed detector bias is more than a small fraction of the signal voltage, it may cause distortion due to a deeply modulated carrier Wave.

Accordingly, it may be stated that it is one of the main objects of the present invention to provide a method of eliminating, or suppressing, inter-channel noises in a receiver equipped with automatic volume control, and wherein the detection and automatic 4volume control functions are performed by a single tube, the method consisting in utilizing the said single tube, in the absence of a signal, to suppress the inter-channel noises by the application of a cut-off voltage to the grid of an audio frequency amplifier stage, such a method of noise suppression being adapt ed to function without distortion, and the presence o-r absence of a signal voltage greater than a noise voltage determining whether the bias on the grid of the first audio frequency amplifier stage is such as to repeat, 0r such as to reject the audio frequency voltage reaching the said stage; it being possible, in addition, to so design the time constant of the cut-off action so that no change in the bias of the said audio stages takes place sufficiently quickly to produce distortion. Another important object of the present invention is to provide in a radio receiver a single stage capable of performing diode detection, delayed automatic volume control and inter-channel noise suppression functions, the noise suppression function being such that the grid of the rst audio frequency amplifier tube is, in the absence of a signal biased to cut-off, but is automatically removed from cut-off when a signal is received.

Still another object of the invention is to provide a radio receiver equipped with a duplex diodetriode tube capable of accomplishing detection, automatic suppression of inter-channel noises, and delayed automatic volume control, the duplex-diode triode tube embodying at least three independent anodes with a common cathode, one of the anodes being used for detection, a second for automatic volume control, and a third anode as a source of direct current potential for biasing a grid of a tube subsequent to the triple anode tube.

Another object of the present invention is to provide a radio receiver wherein a single tube functions to perform detection, automatic volume control and muting, the tube including a triode section independent of at least two ldiode sections, the muting being accomplished by controlling the bias on a grid other than the signal grid of a multi-grid tube following the aforementioned multiple duty tube.

Fundamentally, the present invention contemplates the use of a duplex diode-triode tube as a detector, source of automatic volume control voltages, and additionally, as a special means for suppressing circuit noises. The tube is arranged in such a manner that when zero or very e small signal is present at the detector, the grid of the first tube following the detector shall be biased to cut-off; that is, shall pass no plate current. The circuits contemplate that when a signal is received, provided it is above a certain minimum and predetermined voltage, the tube following the detector (that is, the first audio tube) shall be removed from cut-off, and the signal passed in the usual manner. The removal of the grid from cut-oif is arranged to be accomplished by the supply of a reversed voltage from the rectied current through an impedance in series with one of the sets of diode elements in the duplex diode-triode tube.

The coupling for the audio or signal voltage from the detector to the first audio tube is made by means of a coupling condenser arranged in any suitable manner. Ordinary automatic volume control, or delayed automatic volume control, may be used, and half wave or full wave diode detection and rectification may also be employed. Again, detection and rectification may be from different sets of diode elements, or may be from the same ones. The audio tube control may be of any type ordinarily suitable for audio ampliiication. If the audio tube is a triode the cut-off control should ordinarily be on the control grid. However, if the audio tube is of the multi-grid type, this control may be on the signal control grid, or on the suppressor grid, if the tube is of the pentode audio output type.

Finally, it may be stated that other objects of the present invention are not only to improve generally the simplicity and efficiency of automatic volume control circuits for radio receivers, but to also provide receivers of this type which not only include the aforedescribed features, but are durable and reliable in operation, and economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into eiiect.

In the drawings,

Fig. 1 diagrammatically shows a superheterodyne type of receiver embodying the present invention,

Fig. 2 shows a modification of the invention of Fig. 1,

Fig. 3 shows still another modification,

Fig. 4 shows an arrangement similar to the modification in Fig. 3, a modified form of noise suppression function being shown.

Referring now to the accompanying drawings wherein like reference characters designate the same circuit elements in the different figures, there is shown in Fig. 1, in schematic fashion, a superheterodyne receiver embodying the present invention. As is well known to those skilled in the art, such a receiver usually embodies an antenna circuit, a tuned radio frequency amplifier, a local oscillator, a rst detector stage, an intermediate frequency amplifier, a second detector stage, and one or more audio frequency amplifier stages followed by a reproducer. A unicontrol means is shown for simultaneously tuning the radio frequency amplifier, first detector and local oscillator stages, it being understood that various expediente are known for tuning the receiver throughout the broadcast range while supplying a substantially constant intermediate frequency fiom the output of the intermediate frequency amplier.

The output of the intermediate frequency ampliiier includes a resonant network l which is iixediy tuned to the aforesaid intermediate frequency, the resonant circuit I being magnetically coupled to the resonant input circuit 2 of the second detector, this input circuit also being fixedly tuned to the operating intermediate frequency.

The second detector stage includes a tube 55, which is of the so-called duplex diode-triode type, and is adapted to cooperate with its circuits to function as a combined second detector, automatic volume control, and inter-channel noise suppressor device. The first audio frequency amplifier stage includes a tube .'55 having its anode arranged for connection to an output load circuit which may include additional power amplifier stages and a reproducer (not shown).

Of course, it is to be clearly understood that the present invention is not limited in any manner to a superheterodyne receiver, but may be employed with equal facility in connection with a radio frequency amplifier preceding the tube 55 and an audio frequency amplifier following the latter. Hence, in Figs. 2, 3 and l the symbol R. F. is to be understood as designating a source of radio frequency energy, whether, it be of intermediate frequency, ora frequency usually employed prior to a detector in a tuned radio frequency amplier receiver.

Considering, now, the specific nature of the network connected between the input circuit 2 and the tube 46, it is believed suiiicient for the purposes of this application to conventionally represent the tube 55. As shown, the tube comprises a triode section including a cathode S (of the indirectly heated type), a grid Li, and a main anode 5; a diode section consisting of a diode anode Di and a portion of the cathode 3 not emitting electrons for the main anode 5; and an additional diode section consisting of the diode anode D2 also disposed adjacent the cathode 3 out of the electron stream to the main anode 5. Such a tube has been specifically described and claimed in the aforesaid Shrader application, and the construction shown therein may be employed in designing tube 55.

Brieiiy, in actual construction the tube 55 is provided with a cathode sleeve, within which is mounted the heater element, a grid and main anode being concentrically positioned about the sleeve. The pair of diode anodes Di and D2 are placed around an end of the sleeve projecting beyond the triode grid and anode. In operation, the two diodes and the triode are independent of each other except for the common cathode sleeve which has one emission surface for the diode anodes Di and Dz, and another for the triode grid and anode.

The circuits associated with the electrodes of tube 55 may now be considered. Fig. 1 shows half-wave diode detection, and separate halfwave diode rectification for delayed automatic volume control, together with coupling from the detector diode through a condenser to the grid of the first audio stage tube. The circuit is so arranged that when no signal is applied, the plate current of the triode element is used to bias the contrl grid of the first audio amplifier tube to cut-off, and prevent any relay action by the latter tube.

The incoming signal is applied between the cathode and the diode anode Di, and the potential developed across resistor Ri by the audio component of diode current is coupled by means of the condenser Ci to the grid 6 of the first audio tube. This grid, however, in the absence of a signal above a predetermined threshold, is biased to cut-off by means of the plate current of the tube 55 flowing through the resistor R3. When a signal is received on diode anode D2 by means of the coupling condenser C3, the voltage drop through resistor R2 is such as will bias the grid of the tube 55 to cut-off. This eliminates the potential drop across resistor R3, and removes the audio frequency tube from cut-off.

In other words, when no signal is received, the triode section of tube 55 passes plate current which is used to develop a voltage for biasing the second tube 46 to cut-off. When a signal is received the first tube biases its own triode grid element 4 to cut-ofi, and thus removes the cuteff bias of the second tube. By the proper choice of values for the various resistances and condensers the circuit can be made to operate in a very satisfactory manner. It will be noticed that the diode D2, together with the grid 4 of the first tube, is given a negative bias controlled by the potentiometer C which enables a suitable adjustment of operation to be made. It is, of course, desirable that the audio tube be one which exhibits a sharp cut-off of plate current with change of grid Voltage.

The automatic volume control function is accomplished through the path 1 (designated AVC) connected between the grid circuits of the radio frequency amplifier, first detector, intermediate amplifier stages and one side of the resistor R4. The opposite side of the latter is connected to the diode anode D2. The diode rectifier including anode D2, then, functions to control the bias of the grid 4 for the muting action, and also furnishes rectified current for securing a volume control voltage by' the drop across resistor R4. 'Ihe direct current connections to the controlled pre-detecto-r stages are too well known to require further description. The condensers C4, C5, Ce are radio frequency by-pass condensers. The bleeder resistor provides the electrodes D1, D2, 5, 4 of tube 55 with the required potentials; the grid 6 of tube 45 is given a normal bias, relative to the cathode, in Virtue of the drop across resistors Re, R3, Rs. The adjustable connection 8, between the resistor R2 and the potentiometer resistor C, furnishes a means for adjusting the sensitivity of the receiver at which muting occurs.

Recapitulating the operation of the tube 55, and its associated circuits, the rectified voltage across the resistor R2 occurs when a peak signal greater than the negative bias at the potentiometric resistor C is impressed upon the signal input circuit 2. Automatic Volume control action then occurs, and the grid 4 is biased to cut-off by the additional voltage across the resistor R2. The voltage dro-p across resistor R3 is then Zero, or very small, and the audio amplifier tube 45 is working with normal bias which passes the audio signal. With no signal delivered to the diode anode D2, the grid of the tube 55 is removed from cut-oh', and the increased potential drop across the resistor Rc biases the tube 46 to cut-off. As pointed out heretofore the audio signal voltage is fed to the amplifier tube 45 from the path including resistor R1.

Fig. 2 shows full-wave diode detection with diode bias, and automatic volume control so arranged that when lno signal is received the plate current of the triode section flowing through a resistance biases the control grid of the first audio tube to cut-off, and thus prevents noise from being passed. When a signal is received the grid of the triode section of the tube 55 is biased at, or near, cut-off, and the signal applied through the coupling condenser from the grid of the tube 55 to the control grid of the first audio tube is relayed by that tube.

More specifically Fig. 2 is similar to Fig. 1, except that there is provided full-wave diode detection and automatic volume control rectification from the same elements, and a slightly different method of connections to the first audio tube is given. As before, the rectified current is led off in the usual manner, for automatic volume control, and the audio component is coupled to the succeeding tube by means of the condenser C1. the first audio tube 51 is biased to cut-.off by means of the normal bias and the voltage due to the plate current of the tube 55 fiowing through the resistance R2.

When a signal is received the direct current drop across resistor R1 biases the grid 4 to cut-off, reduces, or eliminates, the current flowing through resistor R2, and removes the grid of the audio tube from cut-ofi, thus passing the signal. This arrangement provides for suppression of noise in that a signal of a predetermined Value is required to bring the tube 55 to cut-ofi, and to remove the audio tube from cut-off. Adjustment of volume may be had by means of a potentiometer C' in that the signal voltage applied to the grid of the audio tube may be varied at will. The circuit shows ordinary automatic volume control; that is, without delayed action.

Briefiy, then, in Fig. 2 the rectified voltage across the resistor R1 brings the grid 4 to cut-off, and removes the audio stage from cut-ofi, functioning in the same manner as in Fig. 1. Full wave rectification occurs between the two diode plates D1 and D2, these two diode plates being connected to opposite sides of the input circuit coil 2". The resistor R1 is connected between the grounded leg of the cathode of tube 55 and the midpoint of coil 2', the condenser C7 functioning as a radio frequency by-pass condenser connected between one side of resistor R1 and the lead 9 between the grid 4 and the automatic Volume control path. The resistor R7 and the condenser C8 provide a filter network for the automatic volume control path.

The condenser C9 provides a radio frequency by-pass around the resistors R2 and Rs. The potentiometer at C provides a manually operable audio volume control, and it will be observed that the tube 51 is of the pentode type including a suppressor grid, disposed between the anode and screen grid, at cathode potential. In the modification shown in Fig. 2 the automaticA volume control is obtained from the voltage drop across the resistor Ri, and, as stated above, this resistor also provides the audio signal for the tube 51, as well as control of the muting bias.

Passing on to Fig. 3, the latter shows halfwave diode detection; separate diode rectification for delayed automatic volume control voltage; and the use of a third diode for the development of direct current for the cancellation of cut-off bias on the grid of the rst audio tube when a signal is received. In the case of Fig. 3 a triple diode arrangement is shown; the grid and plate of the triode of the tube 55 are connected together to serve as a diode anode D3. Detection is secured by means of diode anode D2 which is coupled to the radio frequency input circuit 2 by means of condensers Cz and Ca. The detected audio component is fed to the rst audio tube by means of condenser C1 in the usual manner.

When no signal is received, the grid 6 of When no signal is being received the voltage supplied from across the voltage bleeder points EF biases the grid 6 of the audio tube 46 to cutoff; hence, no circuit noises are passed. An incoming signal rectiiied by diode anode D1, and flowing through the resistance R1, produces a counter-bias which removes the audio tube from cut-oit. Automatic volume control voltage is supplied by the rectified current ilowing through the diode anode D3 and the resistance Ra. This may be delayed by the desired amount by means of the automatic volume control bias from across the bleeder points FH. It will be noted that the potential of the circuit D2, R2 and R4, varies as a unit with the voltage developed across R1 so that this voltage supplies the bias for the audio tube.

summarizing, the operation of the modication shown in Fig. 3, it will be observed that effectively the tube 55 includes within its envelope three independent diodes, two of which diodes include the anodes D1 and D2 while the grid and plate of the tube 55 are tied together to serve as the additional one. The circuit is arranged so that with no signal to the diodes, the bias on the amplifier tube 46 is at, or beyond, cut-off. With signal, the voltage across R1 bucks out part of the applied bias, and removes the amplifier tube from cut-off thus, passing the signal. Detection takes place in the diode having resistance R2, while automatic volume control voltage is developed across R3 when the peak signal equals the delay bias on this diode.

The modification shown in Fig. 4 is substantially similar to that shown in Fig. 3, but with the cut-off bias of the first audio tube applied to the suppressor grid I of the audio tube 51. tube 5'! is of the pentode type including a signal grid 6, a positive screen grid, and the grid I0 disposed between the screen grid and the anode. The suppressor grid I0, a grid other than the signal grid, is connected by a lead I I to the positive side of the resistor R1, and the cut-off bias for the audio tube 51 is applied to the grid II), rather than to the signal grid 6. This is arranged by means of a positive bias of approximately 45 volts, applied to the cathode from the resistance strip of the voltage supply.

In other words, the cathode of tube 5'! is connected by a lead I2 to the positive side of the bleeder. The signal grid 6 of tube 51 is connected through the adjustable tap of the volume control C and lead I3 to a point on the bleeder which is less positive than the point to which the lead I2 is connected. The negative side of the resistor R1 is connected by a lead I4 to a point on the bleeder which is less positive than the point to which the lead I3 is connected.

When a signal is impressed on the resonant input circuit 2, the audio frequency tube 5'! is removed from cut-oit by means of the direct current drop across the resistor R1. The audio voltage from the diode anode D2 is supplied to the grid 5 of the audio tube by means of the condenser C1. With no signal impressed on the input circuit 2, the suppressor grid I!) is 45 volts negative with respect to the cathode of tube 5l. The rectiiied voltage developed across the resister Ri, when a signal is present, bucks the applied voltage, and brings the suppressor grid i?, to zero potential, and passes the signal. Otherwise, the operation of the arrangement shown in Fig. is substantially the same as that described in connection with Fig. 3.

While I have indicated and described several This systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without 5 departing from the scope of my invention as set forth in the appended claims.

What I claim is:

l. In combination, a high frequency amplier whose gain is to be controlled, a multi-grid audio 10 frequency ampliiier, three independent diodes,

a resonant signal input circuit coupling the output of said high frequency amplifier to one of said diodes, means coupling a second of said diodes to said input circuit, a direct current l5 connection between said high frequency amplifier and the cold electrode of said second diode,

a path of low impedance to audio frequency signais connected between one of the grids of said audio frequency amplifier and the third diode, and a direct current connection between the cold electrode of the third diode and a predetermined point of the nrst diode, and a direct current connection between a grid of said audio frequency ampliiier, other than the first mentioned grid, and said rst and third diodes.

2. In an arrangement of the type disclosed in claim l, a common tube envelope, said three diodes being disposed within said envelope and having a cathode common to all oi the diodes.

3. In a radio receiving system, an electron discharge tube provided with a cathode and at least three cold electrodes, a signal input circuit connected between a cathode and one of the cold electrodes to provide a diode rectification network, means for impressing signal energy from said input circuit between the cathode and a second of said cold electrodes, said cathode and second cold electrode providing a diode detection network, circuit elements electrically associated with the cathode and third cold electrode to provide a second diode rectication network, an electron discharge tube having its input electrodes connected to said diode detection network, a transmission control electrode included within said second tube, a connection between said transmission control electrode and said rst diode rectification network, and an automatic volume control connection to said second diode rectification network.

4. In a receiver as dened in claim 3, said second tube being an audio amplier, and said transmission control electrode being a grid disposed between the plate of the audio amplier and its signal input grid.

5. In a receiver as deiined in claim 3, two of said cold electrodes being disposed outside the electron stream from said cathode to the third cold electrode.

6. In a receiver as dened in claim 3, a load resistor included in said iirst diode rectication network, a connection to said transmission control electrode being to a point of positive potential on said resistor, and the cathode of said second tube being at a positive potential with respect to the negative side of said load resistor.

7. In a radio receiver of the type provided with a signal amplier, a diode detection network, and an audio frequency ampliiier, connections between the detection network and the input electrodes of said audio amplifier, a diode rectiiication network coupled to the signal input circuit of said detection network, said diode rectiiication network including a load impedance for developing a direct current voltage depending upon the amplitude of received signals, connections between at least one electrode of said audio amplifier and a point on said load impedance whereby the transmission efficiency of the audio amplifier is decreased when the received signal amplitude is less than a predetermined minimum value, and a second diode rectification network coupled to said input circuit of the diode detector for providing a gain control voltage for the signal amplifier.

8. n a receiver as defined in claim 7, a common tube envelope housing the diode electrodes of said diode detector and said two diode rectication networks.

9. In combination, a. high frequency signal transmission tube whose gain is to be controlle-d, three diode devices, an audio electron discharge tube whose operation is to be controlled, a resonant signal input circuit coupling the first tube output to one of the diode devices, means coupling a second of the diode devices to said input circuit, a gain control direct current connection connected between a gain control electrode of the said first tube and the cold electrode of said second diode, means in circuit with the third diode device for developing an audio voltage from signals impressed thereon from the said input circuit, means for impressing the audio voltage upon said audio tube for utilization thereof, means establishing a cold electrode of the audi-o tube at an abnormally high negative bias with respect to the cathode thereof, and means in circuit with said first diode device for reducing the abnormal bias to a desired normal bias when signals are impressed on said input circuit.

10.1n combination, a high frequency signal transmission tube whose gain is to be controlled, three diode devices, an audio electron discharge tube whose operation is to be controlled, a resonant signal input circuit coupling the first tube output to one of the di-ode devices, means coupling a second of the diode devices, to said input circuit, a gain control direct current connection connected between a gain control electrode of the said first tube and the cold electrode of sai-d second diode, means in circuit with the third diode device for developing an audio voltage from signals 5 impressed thereon from the said input circuit, means for impressing the audio voltage upon said audio tube for utilization thereof, means establishing a cold electrode of the audio tube at an abnormally high negative bias With respect to the cath-o-de thereof, and means in circuit with said first diode device for reducing the abnormal bias to a' desired normal bias when signals are irnpressed on said input circuit, said three diode devices comprising a common cathode and three independent anodes, said electrodes being disposed in a common tube envelope.

1l. In combination, a high frequency signal transmission tube whose gain is to be controlled, three diode devices, an audio electron discharge tube whose operation is to be controlled, a resonant signal input circuit coupling the first tube output to one of the diode devices, means coupling a second of the diode devices to said input circuit,

a gain control direct current connection connecte-d between a gain control electrode of the said first tube and the cold electrode of said second diode, means in circuit with the third diode device for developing an audio voltage from signals impressed thereon from the said input circuit, means for impressing the audio voltage upon said audio tube for utilization thereof, means establishing a cold electrode of the audio tube at an abnormally high negative bias with respect to the cathode thereof, and means in circuit with said first diode device for reducing the abnormal bias to a desired normal bias when signals are impressed on said input circuit, said audio voltage being impressed between the input grid and cathode of the audio tube, and said cold electrode of 40 the audio tube being said input grid.

JOHN S. STARRETT. 

